ROLL QUALITIES AND ECONOMIC UTILISATION OF ROLLS

Umesh Singhal Divisional Manager

Tata Iron & Steel Company Ltd Jamshedpur

1. ROIL QUALTITES

Rolls do the most important work in a rolling mill. They

constitute a very important component of the running cost of the mill.

Therefore it is imperative that optimum performance be obtained from

the rolls.

The roll has mainly three parts -- Wbbblers, Neck and Barrel.

The wobblers are for driving or rotating the rolls, the necks are to

support them in the mill housings and the barrel portion is the

working portion of the roll where actual rolling takes place.

Requirements of a roll:

• Resistance to breakage: The roll should be strong enough to roll materials without breaking.

• Resistance to wear: It should be wear resistant so that maximum rolling can be done with high dimensional accuracy and surface finish.

• Resistance to fire cracking and spalling: The roll surface should not develop fire cracks or spall.

• Good surface finish: It should impart good surface finish to the rolled product.

• Good biting properties: It should be able to bite the rolled stock well and not slip easily.

It is not possible to get all the above mentioned qualities in a

single roll material. Therefore a roll designer has to select the

6.1

best material out of those available to suit the requirements.

Generally it may be said that the harder the rolls, better the wearing

properties, less tougher they are and vice-versa.

2. CLASSIFICATION OF ROLL MATERIALS

Roll aterials fall into three main groups:

(a) Steel base (b) Iron base (c) Special materials like tungsten

carbide and high speed steel

2.1 Steel Base: Rolls

These may be cast or forged.

(a) Cast steel rolls: These rolls have qualities and grain structure

like steel although the carbon content may be quite high (upto

2.5%). The cheapest rolls in this group are plain carbon steel

rolls. To get better wearing properties, molybdenum and chromium

are added and nickel is introduced for imparting strength and

resistance to fire cracking. These rolls with small quantities

of alloys are used in big size mills such as Blooming Mills. The

most used rolls in this group are called 'Adamite' rolls.

Depending upon carbon content these adamite rolls are graded as

A, B, C, D & E, Adamite 'A' being the softest but toughest and

Adamite 'E' the hardest but least tough. All these rolls are

cast, heat treated and machined and have nearly uniform hardness

throughout the transverse cross section.

(b) Forged steel rolls: Rolls are also made by forging. The forged

rolls contain less carbon compared to cast rolls because high

6 . 2

carbon content would cause cracks during the forging process.

The structure of forged rolls is denser than that of cast steel

rolls and therefore, are tougher and can take more load.

However, because of the lower carbon content the hardness is low

and more prone to wear than cast rolls. These rolls are

primarily used where they have to withstand high loads as in

Blooming Mills or in Heavy Section Mills. Forged and hardened

rolls are also used as back-up rolls in 4 high mills although

normally alloy cast steel rolls are used for back-up.

2.2 Iron Base Rolls

These are maximum used in rolling mills mainly because they

impart good finish and posses good wearing properties. Many years ago

cast iron or chill cast iron rolls were only used. To increase the

strength, finish, wearing properties and heat resistance, a number of

alloying elements, are now added to the iron rolls. Also the improved

casting techniques and heat treatment have greatly enhanced the

properties of these rolls. There are a large variety of cast iron

rolls available in the market but only a few representative ones are

dealt below:

(a) Chill cast iron: The roll casting is made vertically with barrel

in chill moulds. The chill surface is very hard and imparts good

finish. These rolls are used for rolling flat products like

sheets and plates and small sections. The addition of molybdenum

increases the strength and heat resistance properties. The

composition is more or less similar to cast iron but its

structure is different. The outer surface of the barrel is

chilled (Fe3C) imparting very high hardness. The thickness of

the chill varies. It may be even upto 20 mm thick depending upon

its application. Beneath the chill zone there is a transition

zone known as Mottle Zone where carbon is gradually flaking from

6.3

a few specks to the full flake. The core portion is called Grey.

Hardness drops down if the chill is worn out.

(b) Nichillite rolls: These are also chill rolls but nickel is added

to them to impart high hardness and strength. The high hardness

provides good surface finish to the product. The hardness

characteristics are same as that of chill rolls. These rolls are

used for flat rolling. In TISCO these rolls are used in the

Finishing Stand of Strip Mill.

(c) Alloy grain rolls: As the name suggests alloy grain rolls are

made from iron having carbon more than 3% with nickel, chromium

and molybdenum added as alloying elements. The free carbon is

present in the form of flakes. These are sand cast rolls and

have reasonably good strength and wearing properties. These

rolls are useful in section rolling mostly in the intermediate

passes where light drafting is possible. The hardness drop from

surface to core is gradual.

(d) Indefinite chill rolls: These are alloy iron rolls and are cast

in chill moulds. After casting the hard top chill layer is cut

off and the remaining part of the roll has hardness practically

constant upto a great depth. In fact drop in hardness from the

surface to the core of the roll is gradual upto 4" to 5" depth

compared to chill roll where the drop in hardness is sharp.

Therefore indefinite chill rolls are used in the intermediate and

finishing stands of Section Mills where deep grooves may have to

be cut to make the required profiles for the sections to be

rolled. These rolls have better resistance to fire cracking and

spalling than the chill rolls and also better strength. They are

reasonably tough with good wearing properties. In TISCO these

rolls are used in the intermediate stands of Strip Mill.

6 . 4

(e) Spheroidal graphite rolls: This is another type of alloy iron

rolls where the structure is completely different from that of

cast iron rolls. The carbon is present in the form of spheroids

or nodules which increases the ductility and makes the roll more

resistant to fracture. Nodularisation is achieved by the addition

of calcium silicide and magnesium or cerium. The nodular

structure of carbon imparts better tensile strength than that of

cast iron along with better wearing properties. However, such

rolls are more prone to fire-cracking and need lot of external

cooling. Due to greater strength such rolls are used to replace

other types of iron base rolls. Their wearing properties are

also better than steel base rolls and at times they are also made

to replace steel base rolls. The hardness drop in such rolls is

minimal. In TISCO these rolls are used in Medium & Light Struc-

tural Mills, Sheet, Bar & Billet Mills, Merchant Mill No.II and

also in 32" Reversing Mills.

(f) Double poured rolls: In order to obtain both high resistance to

wear and high strength, roll makers have developed a roll making

technology wherein the outer shell is made hard and the inner

core tough by double pouring of metal of different compositions.

The shell composition is maintained to give very high wear

resistance properties and the core composition to give more

strength. These are very costly type of rolls.

(g) Centrifugally cast rolls: The composition of centrifugally cast

rolls is more or less similar to that of double poured rolls.

First, metal which gives high hardness and better wearing

properties is poured in the mould, which is then revolved at high

speed. After sometime molten metal of different composition is

poured in for the core of the roll to make it more tough. These

rolls are superior to double poured rolls as the shell portion is

more dense giving better properties. Double poured and centri-

fugally cast rolls are also known as Duplex Rolls.

6.5

(h) Tbingsten carbide rolls: The demands of modern high speed Rod

Mills having No-Twist Finishing Blocks brought about the develop-

ment of special roll materials particularly tungsten carbide as

an ideal roll material. In tungsten carbide rolls, tungsten

carbide is the major alloying constituent which gives the roll

high wear resistance and hardness. The other constituent is the

binding material cobalt although nickel has been increasingly

used in the last few years. The general use of a single carbide

grade throughout the entire finishing block is not always the

optimum solution. Atleast two or more grades must occasionally

be considered. TUngsten carbide roll manufacture is a highly

specialised field involving metallurgical, chemical and

mechanical processes. After selecting the powder composition,

the main stages from powder to finishing roll block are pressing,

shaping and sintering. Sintered roll blanks have a maximum outer

diameter of 500 mm. Once the sintering process is complete

carbide rolls reach their peak hardness and are then more

difficult to machine. Mostly they are ground by the manufac-

turers and supplied to the users to make the necessary roll

passes.

Some Rod Mills also use high speed steel instead of tungsten

carbide for rolls of the No-Twist Finishing Block.

Annexures I & II give the types and qualities of rolls used in TISCO.

3. ECONOMIC UTILISATION OF ROLLS

Rolls are very expensive item of the Rolling Mill. Therefore

proper care in selection and utilisation of rolls is of extreme

importance. If the rolls fail in service or do not give the expected

satisfactory life, it adds upto the cost of production. The cost of

rolls varies from Rs 70/- to Rs 100/- per kg depending upon type and

quality. Wear and breakage of rolls should be avoided as far as

possible.

6.6

Further, the roll breakage not only means a full loss of an

expensive roll, but along with it damage to guides, rest bars, chocks,

driving systems, etc. In the event of a roll breakage there is loss

in production due to mill stoppage for several hours; if the roll

failure occurs in a primary mill it will hold up the Finishing Mills

as no steel can be supplied to these mills and also hot ingots may

have to be put in stock.

To avoid the above losses the rolls should be properly designed

and carefully used.

The roll consumption in mills rolling flat products is always

high mainly due to the following reasons:

o high rolling loads.

o flat products require better surface finish and therefore the roll changing is more frequent.

Rolls consumption can be reduced by some of the following

actions:

• Improving roll quality so that the rolls do not wear fast. In TISCO harder rolls have been introduced in Merchant Mill Finishing Stands, Nichillite Rolls in Strip Mill, SG Iron Rolls in Sheet Bar & Billet Mills (at the 32" Reversing Mills) and Medium & Light Structural Mill (for rolling beams and channels).

• Increasing the working range of the rolls by starting with bigger size rolls as done in TISCO's Blooming Mill No.I, Sheet Bar & Billet Mill Nos.I & II and presently being contemplated also in Merchant Mill No.II.

• Salvaging damaged/scrapped rolls fiich is being done extensively in TISCO. The damaged rolls are salvaged by welding e.g rolls with damaged journals. Several scrap rolls of bigger mills are being machined and converted to rolls of smaller mills and used.

• Avoiding roll failures through improvement in the roll designs and faulty handling.

6.7

3.1 Roll Failures

Roll failures can take place primarily due to two reasons:

(a) Defects in manufacture or design of roll (b) Faulty handling

The manufacturing defects would have to be dealt with, in the

course of a separate lecture on 'The Manufacturing of Rolls'. However

some examples of design defects which we have overcome in TISCO are

mentioned below:

3.2 Design Defects

(a) Neck fillet radius: The neck fillet radius should be

generous so that there is no undue stress concentration.

The fillet radii of roll of several mills in TISCO e.g

Medium & Light Structural Mill rolls, Plate Mill rolls,

Strip Mill rolls have been revised over the last few years.

This has given good results causing reduction in roll

breakages at neck fillet (Figs.1 & 2).

(b) Pass fillet radius: Special attention has to be given to

this also because rolls can break at collar holes if the

filler radius is too small. The fillet radii have been

specified in roll pass layout drawings, to ensure that

during machining of the rolls these fillet radii are

maintained. These fillet radii have been carefully fixed

keeping in view the permissible corner radii at collar.

Further in the finishing passes for angles and channels

where the corners are theoretically supposed to be sharp, a

nominal radius of 1 to 1.5 mm is recommented to minimise the

effect of stress concentration. Both these actions have

resulted in reduced roll breakages (Figs.3 & 4).

6.8

fiG.1

6.9

•

F-7

n

too.

FIG.3

6.11

ANGLE FINISHING PASS WITH 1-R

BEAM FINISHING PASS WITI-1 1.5 R (sharp corners removed to

avoid stress concentration) FIG.4.

(c) Wobbler design: A roll can break at the Wobbler if the

design is too weak. A good example from TISCO is the change

of design of Plate Mill rolls from 5-pod wobbler design to

flat wobbler design (Fig.5). Several years ago the wobbler

design of Sheet. Mill rolls was also changed. Similarly the

palm end design of 40" Blooming Mill Cogging rolls was

changed because of excessive breakages of fork.

(d) Other design aspects of roll: Rolling of sections causing

excessive load on rolls can be a cause of roll breakage e.g

heavy beam sections at Heavy Structural Mill, wider and

thinner sections at Strip Mill. This can be remedied by

intermediate heating of the .stock or avoiding the sections

which become uneconomical due to roll breakages. The pass

design and pass sequence should be made such that the rolls

do not get over loaded.

Some of the design defects cannot be overcome and also it

always may not be possible to make changes in design. Under

the circumstances, roll material should be changed and if

possible a stronger material used instead, sacrificing a bit

of wearing properties e.g in TISCO, some forged steel rolls

were being used instead of cast steel rolls at 28" Reversing

Mill of Medium & Light Structural Mill for rolling beams and

alloy steels and SG iron rolls are being used in place of

indefinite chill rolls in intermediate stands of 24" mill of

Medium & Light Structural Mill for rolling beams and

channels.

3.3 Faulty Handling

Damage to the rolls can be avoided by using them carefully and

avoiding mishandling. The following factors are responsible for

breakage of rolls during use. Rolls becoming weaker during use.

There are three main causes:

6.13

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FIGS

(a) Fatigue: In every revolution during rolling the stress in the

rolls get reversed and this continuous reversal of stresses leads

to fatigue and over long period initiates cracks. Fatigue cracks

are even visible and suitable action should be taken to remove

such cracks before further using the roll or discard the roll to

avoid mill delay due to breakage.

(b) Fire cracks: These are caused by alternate heating and cooling

of rolls by accidental stoppage of cooling water when the roll

surface becomes hot suddenly and fire cracks appear when the

cooling water circulation is re-established. If such a situation

OMITS, the rolls should be allowed to cool down slowly before

re-starting the water circulation. Fire cracks also result due

to over heating of the rolls. They have a tendency to go deeper

with use. The cooling water can get entrapped in the crevices of

fire cracks and turn into steam; the pressure thus generated

aggravates making the crack deeper and wider and the situation

becomes worse with alternate bending of the rolls during rolling.

Efforts, therefore, should be made to remove the fire cracks by

dressing when they are shallow in order to make the roll healthy

again. Ideally, the roll should be dressed as soon as the fire

cracks are detected.

(c) Roll diameter wear: After every rolling the rolls are dressed

and the diameter becomes smaller making the rolls weaker. Added

to that is the effect of fatigue cracks and fire cracks. The

smaller rolls should, therefore, be carefully inspected before

reuse and should be scrapped if their diameters becomes 10% below

the nominal size of the mill.

(d) Roll fixing in the mill: Roll bearings and driving coupling play

vital part in getting full life out of rolls:

6.15

o The roll bearings and chocks should be properly maintained. If the chocks play in the housings or if they are not properly located or if the bearings are loose, the chocks or the rolls will jump with every bar rolled resulting in shock load on the roll which can lead to roll failure. The shape of the bearings should match the shape of the roll neck especially in the taper roll neck bearing. The inner races of roller bearings and the diameter of journals should properly match.

o Where fabric bearings are used the water circulation should be perfect. If the fabric bearings do not get cooling water they would get burnt and cause damage also to the roll necks. Such roll necks would require to be repaired and polished before reuse.

o The roll driving couplings should be properly main-tained. Too much play damages the roll wobblers. The alignment between the driving couplings and the rolls should be as good as possible to avoid breakage of roll wobblers.

(e) Overloading of rolls: This is a major cause of roll breakage.

Most of the mills do not have stress gauges or any other measur-

ing or recording devices for mill loads. The overloading of mill

rolls does not always show up on the motors particularly where

several stands have a common drive. In such cases many a time it

happens that the roll breaks without showing any sign of over-

loading on motors. The rolls may break suddenly by bending at

the time of bar entry without any effect on motor load. The

overload can be due to several factors:

o Entry of double bar, folded bar, forging body or 'tongs' on bar or split end of bar.

o Overdrafting which may cause over-loading and generally takes place where adjustments are made with every pass e.g in Reversing Mills. Overdrafts are also possible when the previous stand roll is broken in a Continuous Mill -- sometimes two or three rolls have broken at a time due to this reason.

6.16

• Cold Bar -- Cold bar needs much more power to reduce and may cause a roll failure. Cold bar may also results when there is a delay in rolling schedule. Quick decision will have to be taken whether the bar temperature is sufficient for rolling. Particular in the night time the bars look hotter than during the day time, which may lead to a wrong judgement and result in more failures during night shift.

• Collar -- When the bar does not get out from between the rolls but due to some reasons goes round the roll it is said to have 'Collared'. This is a definite cause of roll breakage and often damages other Mill equipments. Collaring is due to guide failure or split end or pipy bars.

• Under Feeding -- This means that the bar has entered in a place other than the roll pass and often results in roll failures.

• Rolls or Passes not Changed in Time -- If the pass is not changed in time it gets unduly worn out and also the fire cracks penetrate deeper. The undue wear in the pass will also not give the designed shape from that pass. When a pass is not changed in time, apart from the bad section, the next pass also gets spoiled requiring heavy dressing and thus losing roll life; often collaring may result due to this. Therefore, even though extra production may be obtained from worn out passes, this will be at the expense of roll life because such passes will need more dressing. Besides, the rolls will be put to undue risk during rolling due to the presence of deeper fire cracks. In the case of Hot Sheet Mills if the hot mill rolls are not changed in time they become hollow and consequently run hot to get the sheets of proper shape. When the roughing rolls are worn out there is a tendency to press the screw more to get the required thickness. This generally causes spalling of the rolls.

(f) Cooling of rolls: Rolls should not be abruptly heated or cooled.

The roll cooling should be to such an extent that the roll

surface temperature is about 80°C. To get uniform cooling action

the rolls should be rotated slowly even if the mill is down for

some period. At any time during rolling one should be able to

stop the rolls and touch the surfaces with bare hands indicating

6.17

that the roll has been sufficiently cooled. During rolling,. due

to contact with the rolled material and simultaneous cooling of

surface by water, the temperature inside the roll would be more

than the surface temperature.

As the hot bar touches the roll the temperature at the contact

point immediately rises to that of bar temperature which could be

1000°C, only for a split second. In slower mills the bar remains

in contact with the rolls for a longer period compared to that in

high speed mills. The water cooling problems are more acute in

slower mills. During rolling a part of the roll in contact with

the bar attains a high surface temperature; the cooling water

which comes in contact with this surface vaporizes and forms a

layer of steam on the roll surface which thus gets covered by a

blanket of steam and the cooling water becomes ineffective. Some

high pressure jets should be made to impinge on the roll at the

point where the bar leaves the roll. This will take away the

steam being formed on the hot roll and expose the roll surface

for cooling, the roll can then be cooled by the ordinary water

sprays.

Cooling of bottom rolls is more difficult because on the bottom

roll the water jets have to work against gravity and the water is

not able to reach the full periphery of the roll as in the case

of top roll. Moreover, the bottom jets often get choked due to

the scales generated in the rolling process. Added to this the

rolling crew is unable to see the cooling jets of the bottom

rolls easily, unless special efforts are put in.

Whatever be the cooling process, the surface temperature changes

from 1000° to 80°C during every rotation of the roll, as a result

of which fire cracks develop on the roll surfaces. Special roll

cooling boxes for flat product mills have been tried in some

6.18

places with encouraging results with respect to roll wear and

roll life.

The above is applicable to water cooled rolls. However, the

cooling system for different types of rolls may be different. In

case of chill rolls of Hot Sheet Mills, having to run without

cooling, if water is used, the sheets will get cold and hot

rolling will not be possible. We cannot reheat the packs to high

temperatures because the edges will burn. Therefore the rolling

has to be done without water cooling. To avoid sudden heating of

rolls, the rolls are pre-heated by electric induction. The chill

roll consists of three parts -- top chill (about 10 mm deep),

mottle (about 25 mm deep) and grey iron. All these three

components have different co-efficient of expansion. If the roll

is heated suddenly, the chill expands much more than the mottle

or the grey iron and thus a crack is formed inside the rolls

causing a breakage subsequently.

Hot mill chill rolls are cooled by a mixture of steam and air.

These rolls need cooling when their temperature exceeds the pre-

determined rolling temperature suiting the contour of the rolls

so that the flat products rolled are of uniform thickness. It is

important that not a drop of water should be present in the steam

otherwise rolls which are at high temperature will suddenly

break.

4. MAINIINAhtE OF 11,1,CSTEN CARBIDE ROT LS

Thermal fatigue has got a very harmful effect on roll materials

used for hot working conditions. In cases cracks which combined with

corrosion and abrasive wear, threatens the service life of the pass.

Although the thermal conductivity of cemented carbide is twice that of

6.19

steel, it is still equally important that efficient cooling is

provided.

The phenomenon of thermal cracking or fire-cracking in the roll

pass is inevitable, nevertheless its progress can be checked and its

effect minimised provided following steps are taken.

(a) Re-grinding: Correct and timely regrinding of carbide rolls is

essential if the rated service life is to be achieved. To

minimise and avoid roll failures caused by cracks in the pass, it

is imperative to remove the cracks specially before the rolls are

put back into the mill again. The problem is however to know

when thermal micro-cracks have been removed from pass surface.

Should there any residual cracks after the regrinding process,

the next rolling campaign will start of with faulty rolls which

factor equally increases the risk of roll breakage.

Todate there are no reliable instruments for measuring the actual

depth of micro-crack. But there are some methods and instruments

in the market that can be used to locate their position -- such

as eddy current, magna flux, die penetration, etc. Other

problems may accrue with grinding tungsten carbide rolls in

unsuitable machines, when using the wrong grinding wheels. There-

fore great care and vigilance is necessary.

(b) Corrosion: In many mills, corrosion is a hidden factor in the

deterioration of pass form, because pitting formations are

blurred or worn away by other destructive actions. Many mills

experience periodic 'fluctuations in the cooling water with pH

values dropping below 7, the point at which cobalt grades begin

to corrode. The nickel base carbide grades can be used at pH

values as low as 5 as these are corrosion resistant.

6.20

(c) Overfilling: Overfilling of the roll pass results in increase of

the rolling level which combined with the deterioration of the

roll pass will inevitably lead to roll failure. Therefore over-

filling should be avoided.

(d) Cooling: The object of roll cooling is to retard the formation

of thermal fatigue cracks and surface deterioration. By adequate

and proper roll cooling the roll life in certain mills is

reported to have increased by more than 500. The distribution of

flow of cooling water has a decisive impact on roll performance.

(e) Carbide grade: Correct choice of carbide grade is necessary for

obtaining optimum roll life.

6.21

Annexure-1

PHYSICAL PROPERTIES OF ROT JS

Material/type

Si. No.

Tensile strength elong.

Hardness (deg.Sho)

(kg/ mm2)

1. Indefinite chill/grain 20 to 25* 2. S.G.iron (pearlitic) 40 to 55 3. S.G.iron (accicular) 50 to 60 4. Double poured grain - Shell 15 to 20

Core 25 to 35 5. Cast steel/steel base/

alloy cast steel 50 to 90

0.20 to 0.60* 40 to 75 1.0 to 5.0 40 to 70 1.0 to 4.0 60 to 75 0.1 to 0.3 65 to 90 0.2 to 0.4 35 to 45 1.0 to 10.0 30 to 50

* depending on the type and composition of roll

APPROXIMATE COST OF SOME OF THE ROT J S USED IN TISCO's M1TJS (Prices are based on quotations for 1994-95 -- excluding taxes, transport, etc.)

Sl. No.

Mill Size of roll Quality Wt. in tonnes

Price (1994-95)

Rs

1. 40" Bl.Mill 1016x2286 P.B.(Tayo) A.S 19.00 14,00,680.00 2. SB&B Mill 32" rev. -

850 dia x 1800 P.B S.G 10.40 5,50,935.00 3. SB&B Mill 24" mill -

650 dia x 1066.8 P.B Adam.'C' 3.47 2,60,295.00 4. M&LS Mill 790 dia x 1220 P.B Adam.'C' 5.95 3,85,030.00 5. M&LS Mill 790 dia x 1220 P.B S.G 5.60 3,35,415.00 6. M.Mill No.2 370 dia x 800 P.B I.0 0.90 75,000.00 7. Strip Mill 400 dia x 457.2 P.B D.P 0.75 84,772.00 8. Sheet Mills 815 dia x 1120 P.B Mo chill 7.11 3,34,240.00 9. Sheet Mills 840 dia x 1320 P.B

3 high rgh. T/B A.S 8.64 5,23,060.00

10. Hot Strip 1050 x 1700 P.B D.P indef. 16.23 12,24,990.00 Mill rgh. std. chill alloy

iron (with S.G core)

11. Hot Strip 710 x 1700 P.B Duplex iron 7.85 5,89,380.00 Mill fin. std. indef. chill

(centrifuga- lly cast - with S.G core)

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CO

uonTsodwoo TeoTwaqo Teo -041

z

a4

CID

Roll

qua

lity